A solar cell has an active semiconductor structure including two semiconductor layers in facing contact with each other at a semiconductor junction. When illuminated by the sun or otherwise, the solar cell produces a voltage between the semiconductor layers and thence between a front side and a back side of the active semiconductor structure. (“Front side” refers to the side facing toward the sun, and “back side” refers to the side facing away from the sun.) Advanced solar cells may include more than two semiconductor layers and their respective pairwise semiconductor junctions. The various pairs of semiconductor layers of the advanced solar cells are tuned to the various spectral components of the sun to maximize the power output of the solar cell.
Electrical contacts are applied to the front side and to the back side of the solar cell. The back electrical contact normally is a continuous electrically conductive layer deposited across all or most of the entire back side of the active semiconductor structure, inasmuch as the back side faces away from the sun during service. The front electrical contact normally includes a plurality of interconnected current-gathering strips deposited upon the front side of the active semiconductor structure. At discrete locations, attachment pad regions are defined on the strips so that external electrical leads may be affixed to the front electrical contact.
Solar cells are used in space and terrestrial applications. Particularly for space applications where the solar cells may be inaccessible for many years, and go through many thousands of sunlight/shade (i.e., heating/cooling) duty cycles without any maintenance, the solar cells must be highly reliable. If the structure and performance of any element of the solar cell degrade during service, the power output of that solar cell may be permanently reduced and eventually lost.
During service, the temperature of the solar cell is elevated above the temperature of the ambient surroundings. Some advanced solar cells, such as concentrator solar cells where multiples of the sun power are concentrated on the face of the solar cell by a mirror, reach temperatures of 80-140° C. in current designs. Over periods of 15-25 years and thousands of duty cycles, diffusional processes may cause progressive degradation of the structure of the solar cell that eventually leads to a decrease in performance.
Operable solar cells are known, but there is an ongoing need for an approach to increase the reliability of existing types of solar cells and to achieve high reliability in future types of solar cells. The present invention fulfills this need, and further provides related advantages.